In toner printing a pattern of toner particles is formed and transferred to a receiver. The transferred toner particles are then fused to create adhesive bonds between the toner particles and between the toner particles and the receiver. In most commercial applications, fusing is performed using a process known as contact fusing. In a contact fusing system, the pattern of toner particles and the receiver are passed through a nip between a heated roller and a pressure roller. The heated roller and the pressure roller are biased toward each other and press the pattern of toner particles and the receiver together while the heated roller heats the toner particles and the receiver. The pressure and heat applied during fusing creates the adhesive bonds that form a fused toner image that is bound to the receiver.
Adhesive bonds also arise between the toner particles and the heated roller during contact fusing. Where the adhesive bonds between the toner particles and the heated roller are weaker than the adhesive bonds between the toner particles within the toner image and the adhesive bonds between the toner particles and the receiver, the toner particles separate from contact with the heated roller, remain on the receiver, and cool to form the fused toner image. However, where the adhesive bonds between the heated roller and the toner particles are stronger than the adhesive bonds between the toner particles in the toner image or when the adhesive bonds between the heated roller and the toner particles are stronger than the adhesive bonds between the toner particles, and the receiver, toner particles can separate from the toner image and adhere to the heated roller. This is known as toner offset. Toner offset creates unwelcome artifacts in the toner image being fused by removing toner necessary for the toner image that is being fused. Further, the toner that remains on the heated roller creates unwelcome artifacts in subsequently fused images by transferring to later toner prints or by forming relief patterns in such later formed toner images.
In some toner printers, elongated belts are used for fusing that have the effect of reducing toner offset. One example of this is described in U.S. Pat. No. 5,256,507 (issued Oct. 26, 1993, in the name of Aslam et al). As is described in the '507 patent, an elongated web is heated to fuse the toner image and then cooled to facilitate ready separation of the receiver member with the toner image fixed thereto from the elongated web. The elongated web arrangement also serves to increase the glossiness of the toner image. As a result, this arrangement is particularly useful for multi-color toner image fusing.
Alternatively, other toner printers apply a fusing oil to the heated roller in order to reduce the adhesion between the heated roller and the toner. However, the use of such oil creates new press operating requirements by requiring additional handling of the oil and by requiring procedures and equipment to ensure that oil is applied in a consistent manner. Additionally, at least some of the fusing oil can transfer from the heated roller onto the print creating a print having image quality and handling problems.
In another alternative, toner printers have been developed that use toner particles that incorporate a wax. During fusing such toner particles are heated at least to a glass transition temperature of the toner and to an incorporated melting temperature of the incorporated wax. This causes the wax to liquefy and to separate from the pattern forming material to form a slip layer between the toner particles and a heated fuser roller. The slip layer reduces extent of adhesive bonds between the heated fuser roller and the toner particles and lowers the likelihood of toner offset. However, after fusing, the wax remains on the toner image and creates gloss and image density variations that can lower the perceived quality of toner images made using toners of this type. This is a particular problem with high gloss images that require high fusing temperatures.
One alternative approach is to remove wax from the toner image during fusing. For example, JP2005043532A entitled: “A fixing apparatus and an image forming device” describes a fixing apparatus having a heating roller wherein any surplus amount of wax is removed from the toner image by being drawn into pores in the heating roller. Similarly, JP2006091146A entitled: “An Image Forming Device and a Fixing Apparatus” describes toner image is formed using the toner containing a resin binder, a coloring material and the wax for improving the releasability. In these publications a wax bearing toner is transferred onto a recording sheet and the toner is fixed by a fixing device under heat and pressure. The fixing device has a heating roller in the form of a hollow cylindrical member made of a metal and has a large number of pores extending from the peripheral face of the heating roller and to the hollow part thereof.
According to the '532 publication, when toner is heated, the melting wax forms a layer and is drawn into the pores by capillary action and removed. The wax is absorbed by a glass fiber layer formed inside the heating roller and held. The '532 publication further suggests that since the excess of wax is removed from the surface of the toner image, the gloss unevenness is restrained without making the toner image remarkably highly glossy even when the toner image is suddenly cooled after fixing.
Another approach is shown in JP2005266079A entitled: “Image Forming Apparatus, Wax Removal Device and Image Forming Method”. The '079 publication describes the use of a wax removal part that allows a blade to contact the surface of a recording medium that is at a temperature range not lower than the melting point of the wax included in toner and lower than the melting point of the toner material. The blade removes the melted wax on the surface of the recording medium. A distance between the fixing device and the blade is determined so that the recording medium causes a temperature drop in accordance with the conveyance and the temperature of the surface falls into the temperature range.
Another publication, JP 2002-091205A entitled: “Image forming apparatus” describes another printer with a wax removal system. In one embodiment the wax removal system has a rolling-up (continuous) type web cleaning device and a film anchorage device that positions the web for cleaning. According to the '205 publication, the wax on a recording medium can fully be cleaned by placing a web on a cleaning roller and rolling the cleaning roller in a direction that is the reverse of a direction of movement of a recording medium. The web can be a porous body material which comprises a natural or natural fibrous body or polyester, polypropylene, polyethylene, etc. However, other webs can be used.
The '205 publication also notes in order to acquire a picture without the further loss of density and gloss caused by wax, the cooling temperature in an exfoliation point is lower than the softening temperature of this recording-medium resin, and it is desirable that it is higher than the melting point of a wax. The '205 publication further notes that it will become granular (the wax which began to melt from a toner in this intermediate transfer body and this recording-medium interface) and will adhere on this recording medium after exfoliation if it exfoliates at a temperature lower than wax melting point temperature under the state where this intermediate transfer body and this recording medium touch.
In general then, the approaches of the '507, '146, '079 and '205 publications attempt to resolve the wax problem by cleaning wax from the surface of the toner image. However, it will be appreciated that attempting to fully clean wax from the surface of a toner image can create a risk of damaging the toner image as generally such cleaning processes involve cleaning structures that are held against the toner image while applying cleaning forces to remove the wax from the toner image. Such cleaning processes pose a particular risk of damaging portions of the toner image that have significant variations in toner stack heights such as regions of high density color where many different types of toner are applied or in regions where toner is applied to build toner stack heights that are high enough to create tactile effects.
The risks of damaging the toner image are particularly acute when such cleaning is performed when the toner is at an elevated temperature. Yet in each of the '536, '136, '079 and '205 publications wax removal is performed when the wax is heated to a temperature sufficient to liquefy the wax. As the wax is in intimate contact with the toner image, this necessarily involves removing wax when the toner image is at an elevated temperature and is more vulnerable to damage.
For example, in the '536 and '136 publications, wax is cleaned at the fusing nip while the wax is in a liquid form and the toner is at or above the glass transition temperature for the toner. These in-the-nip cleaning approaches can be compromised by the risk that the fusing process will interfere with the wax cleaning process, and by the risk that the wax cleaning process will reduce the effectiveness of the fusing process. These in-the-nip cleaning approaches further require the use of complex heating roller designs that are capable of removing such wax while also providing heat and pressure to the toner image in the nip.
Similarly, in the '079 publication and the '205 publication, the toner image is allowed to cool below a glass transition temperature for the toner but while the wax is heated above the melting temperature of the wax. As an initial matter, these approaches are only useful for toners that have wax components with wax melting temperatures that are below a glass transition temperature of the toner. Further, these approaches risk damaging the toner image because they require the application of cleaning forces to the toner image when the temperature of the wax is above a melting temperature of the wax and the temperature of the underlying toner is at or close to the same elevated temperature.
What is needed in the art therefore are new methods, fusing systems and printers that enable a toner image to be formed using a toner with a wax while also managing the presence of any such wax on the toner image to eliminate density and gloss variations that without creating damaging the toner image.